Conventionally, as a storage battery system used in relatively severe temperature environments, a stationary storage battery system is known. The stationary storage battery system is normally used by connecting a plurality of batteries in parallel. However, in a structure in which a plurality of batteries is connected in parallel, each battery connected in parallel is often placed in a different temperature environment. When placed in such a different temperature environment, the deterioration rate may differ from battery to battery so that the variation in deterioration between the batteries are facilitated with the result that the overall lifetime of the stationary battery system becomes short. On the other hand, in order to place each battery in the same temperature environment, performing a temperature control to each storage battery may be considered. In this case, however, a temperature control system with each storage battery would be necessary.
In contrast, for example, in Japanese Patent Application Publication No. H 7-153498, it is disclosed that, in the storage battery system consisting of two batteries connected in parallel, when a temperature difference is occurring between batteries, in order to suppress the variations in deterioration between batteries, the battery of low temperature is first charged, and after completion of the charging operation with the battery of low temperature, the high-temperature battery will be charged.
However, in the prior art described above, while the temperature difference is occurring between batteries, only the timing of battery charging is controlled, and since no consideration is given to the deterioration rate attributable to energy amount of storage battery, there is a problem that variations in deterioration between batteries may not be sufficiently resolved.